U.S. patent application number 12/221679 was filed with the patent office on 2010-02-11 for thermocouple vacuum gauge.
Invention is credited to Heinz Ploechinger.
Application Number | 20100034236 12/221679 |
Document ID | / |
Family ID | 41652918 |
Filed Date | 2010-02-11 |
United States Patent
Application |
20100034236 |
Kind Code |
A1 |
Ploechinger; Heinz |
February 11, 2010 |
Thermocouple vacuum gauge
Abstract
A thermocouple vacuum sensor is provided, the thermocouple being
surrounded by a gas or mixture of gases the pressure of which is to
be measured. Cyclically the thermocouple is heated until its
temperature reaches an upper temperature threshold. The
thermocouple is subsequently cooled until its temperature reaches a
lower temperature threshold. The heating time required to heat the
thermocouple from the lower to the upper temperature threshold is
measured. The cooling time required to cool the thermocouple from
the upper temperature threshold to the lower temperature threshold
may also be measured. The pressure surrounding the thermocouple may
then be determined as a function of either the heating time, or the
cooling time, or both.
Inventors: |
Ploechinger; Heinz;
(Freinberg, AT) |
Correspondence
Address: |
PLOECHINGER HEINZ
HINDLNG 68
FREINBERG
A 4785
AT
|
Family ID: |
41652918 |
Appl. No.: |
12/221679 |
Filed: |
August 6, 2008 |
Current U.S.
Class: |
374/45 |
Current CPC
Class: |
G01L 21/14 20130101 |
Class at
Publication: |
374/45 |
International
Class: |
G01L 21/14 20060101
G01L021/14 |
Claims
1. A system for measuring pressure in a partial vacuum comprising:
a thermocouple sensor located within the partial vacuum, the
thermocouple sensor comprising at least one thermocouple which
generates a thermoelectric voltage; a heating circuit operatively
connected to the thermocouple sensor, the heating circuit being
configured to apply heating pulses to the thermocouple sensor; a
voltage sensing circuit operatively connected to the thermocouple
sensor, the voltage sensing circuit being configured to measure the
thermoelectric voltage generated by the at least one thermocouple;
a control processor operatively connected to the voltage sensing
circuit and the heating circuit, wherein the control processor
enables heating pulses during a heating period when the
thermoelectric voltage is below a lower threshold and keeps the
heating pulses enabled until the thermoelectric voltage exceeds an
upper threshold; disables heating pulses during a cooling period
when the thermoelectric voltage exceeds the upper threshold and
keeps the heating pulses disabled until the thermoelectric voltage
falls below the lower threshold; measures the duration of the
heating period, or the cooling period, or both; determines the
pressure of the partial vacuum as a function of the duration of the
heating period, or the duration of the cooling period, or both.
2. A system for measuring pressure in a partial vacuum as in claim
1, wherein the duration of the heating period is measured by any
combination of the following: a) counting the number of heating
pulses within a measuring cycle; b) recording the aggregated
heating time of all heating pulses in a measuring cycle; c)
recording the total duration of the heating period.
3. A system for measuring pressure in a partial vacuum as in claim
1, wherein the power of heating pulses applied to the thermocouple
sensor increases over time throughout the heating period.
4. A system for measuring pressure in a partial vacuum as in claim
1, further comprising a memory operatively connected to the control
processor, wherein stored in the memory is data associating the
duration of a heating period or the duration of a cooling period
with the pressure of the partial vacuum.
5. An electronic circuit for measuring the pressure of a partial
vacuum surrounding a thermocouple sensor comprising: an amplifier
configured to amplify a thermoelectric voltage generated by the
thermocouple sensor; a first comparator configured to compare the
amplified thermoelectric voltage against a lower voltage threshold;
a second comparator configured to compare the amplified
thermoelectric voltage against an upper voltage threshold; a switch
configured to turn on and turn off heating to the thermocouple
sensor; wherein the heating is turned on responsive to a signal
from the first comparator indicating that the amplified
thermoelectric voltage is below the lower voltage threshold, and
the heating is turned off responsive to a signal from the second
comparator indicating that the amplified thermoelectric voltage is
above the upper voltage threshold.
6. The electronic circuit as in claim 5, further comprising: a time
measuring circuit configured to measure at least one of a) the
duration of a heating period during which heating to the
thermocouple sensor is turned on, and b) the duration of a cooling
period during which heating to the thermocouple sensor is turned
off.
7. The electronic circuit as in claim 6, further comprising: a
processor configured to provide a signal indicative of the pressure
surrounding the thermocouple sensor as a function of the duration
of the heating period, or the duration of the cooling period, or
both.
8. The electronic circuit as in claim 7, further comprising a
memory operatively connected to the processor, wherein the pressure
signal generated by the processor is calculated using data stored
in the memory, the data being characteristic of the relationship of
the duration of the heating period or the duration of the cooling
period and the pressure surrounding the thermocouple sensor.
9. The electronic circuit as in claim 8, wherein said
characteristic data is organized as look-up table or a
characteristic line.
10. The electronic circuit as in claim 7, wherein the pressure
signal is provided as a pulse-width modulated signal.
11. The electronic circuit as in claim 5, wherein the heating
consists of applying electric power pulses to the thermocouple
sensor.
12. The electronic circuit as in claim 11, wherein the power
applied to the thermocouple sensor is modulated by alternating the
duty cycle, or the amplitude of the power pulses applied to the
thermocouple sensor.
13. The electronic circuit as in claim 6, wherein the power applied
to the thermocouple sensor during a heating period increases over
time.
14. A method for measuring pressure at partial vacuum comprising
the steps of: (a) providing an electrically generated heating
current to a thermocouple sensor which alternates between an on
state and an off state; (b) measuring a thermoelectric voltage
generated by the thermocouple sensor; (c) comparing the
thermoelectric voltage to a first and a second reference signal;
(d) varying the heating current so as to cause the thermoelectric
voltage to cycle between the first and the second reference signal;
(e) measuring at least one of the rise time and the fall time of
the thermoelectric voltage within a cycle; and (f) determining the
pressure at the partial vacuum as a function of the measured rise
time or fall time or both.
15. A method for operating a thermocouple sensor in partial vacuum
comprising the steps of (a) alternately heating the thermocouple
sensor from a lower threshold temperature to an upper threshold
temperature and (b) cooling the thermocouple sensor from the upper
threshold temperature to the lower threshold temperature and (c)
determining the pressure in the partial vacuum as a function of the
time required to complete step a) or step b).
Description
TECHNICAL FIELD
[0001] The present invention generally relates to a thermocouple
apparatus to measure pressures in a partial vacuum, and more
particularly, to a thermocouple apparatus using a time-measurement
operating method to measure pressure over a wide range.
BACKGROUND OF THE INVENTION
[0002] Thermocouple measuring tubes are known in the art to measure
pressure in a partial vacuum. A change in pressure in the measuring
tube changes the molecular collision rate and therefore the thermal
conduction of the gas or gas mixture surrounding the thermocouple.
Such measuring tubes are typically operated using one of two
alternative operating methods.
[0003] In a first operating method, an alternating voltage is
continuously applied to the thermocouple, thereby heating the
thermocouple. The resulting temperature shift depends on the
pressure of the surrounding gas or gas mixture and causes a change
in the thermocouple's DC output inversely with pressure changes.
Unfortunately this operating method can only be used within a
relatively small pressure measurement range. Examples utilizing
this operating method are DV-4 and DV-6 thermocouple vacuum gauge
tubes manufactured by Teledyne Hastings.
[0004] In a second operating method, the thermocouple's temperature
is electronically controlled to maintain a predetermined
temperature by continuously adjusting the electrical power applied
to the thermocouple. The amount of power applied to the
thermocouple is evaluated and used as a measure for the pressure of
the partial vacuum. This operating method extends the measurement
range of pressures in a vacuum, but not by much.
[0005] U.S. Pat. No. 4,579,002, hereby incorporated by reference
thereto in its entirety, describes an operating method, in which a
pulsed heating current is supplied to the thermocouple. In the
"off" periods of the heating pulse the generated thermoelectric
voltage (EMF) is measured using an amplifier. A comparator compares
the measured, amplified thermoelectric voltage to a set-point
value. If the amplified EMF deviates from the set-point value, the
length of the heating pulse is adjusted to move the amplified EMF
towards the set-point value. This maintains a constant temperature
at the thermocouple while operated.
[0006] A similar method is described in U.S. Pat. No. 5,351,551, in
which the constant temperature at the thermocouple is maintained by
controlling the current of the heating pulse. U.S. Pat. No.
5,351,551 is hereby incorporated by reference thereto in its
entirety.
[0007] U.S. Pat. No. 6,727,709, hereby incorporated by reference
thereto in its entirety, describes a thermal conduction vacuum
gauge using a Peltier tip. The Peltier tip is part of a measuring
bridge in a vacuum chamber. The measuring bridge may be operated at
constant power or at constant temperature. A voltage signal
obtained from the measuring bridge is a measure of the
pressure.
[0008] DD 249 534 A1 describes a method and a system for measuring
the pressure of gases. The partial vacuum is measured by use of a
current-carrying electrical measuring resistor whose resistance
changes with temperature. The measuring resistor is heated by
high-voltage pulses. During the "off" periods of the heating pulse,
low voltage is applied to the measuring resistor in order to
determine its resistance upon cooling. The cooling time following a
heating pulse is used as a measure of the pressure. A disadvantage
is that the low voltage applied during cooling influences the
cooling time.
[0009] EP 1 409 963 B1 by the same applicant, which is hereby
incorporated by reference thereto in its entirety, describes
sensors and methods for detecting measurement variables and
physical parameters. Sensors, for example Pirani measuring
elements, whose electrical resistance changes as the result of
current flow and the associated temperature increase, are supplied
with electrical pulses. The amplitude of these pulses is changed
over time according to a mathematical function.
[0010] During a measuring cycle the voltage of the pulses may, for
example, be increased linearly over time. Correspondingly, the
power to the sensor measuring element increases quadratically over
time. For a Pirani gauge operated in this manner, the time required
to reach a specified temperature threshold value is used as a
measure of the pressure. Since time can be measured very precisely
at low cost, this method has major advantages, such as extension of
the measurement range, greater accuracy, and low power consumption.
The described operating method is used in model VSP-62 Pirani
vacuum gauges manufactured by Thyracont Vacuum Instruments
GmbH.
[0011] However, the described vacuum gauges only evaluate the time
required to reach a specified temperature during heating. The
cooling time is not evaluated, since measuring the resistance of
the Pirani element requires applying a measuring voltage, which
interferes with the cooling of the element.
[0012] Therefore, in light of the problems associated with existing
approaches, there is a need for improved systems and methods for
accurately measuring pressures in a vacuum over a wide range.
SUMMARY OF THE INVENTION
[0013] In one aspect of the present invention a thermocouple vacuum
sensor is provided, the thermocouple being surrounded by a gas or
mixture of gases the pressure of which is to be measured.
Cyclically the thermocouple is heated until its temperature reaches
an upper temperature threshold. It is subsequently cooled until its
temperature reaches a lower temperature threshold. The temperature
of the thermocouple may be determined in a voltage sensing
electronic circuit by evaluating the thermoelectric voltage
generated by the thermocouple. The heating time required to heat
the thermocouple from the lower to the upper temperature threshold
may be measured. The cooling time required to cool the thermocouple
from the upper temperature threshold to the lower temperature
threshold may also be measured. The pressure surrounding the
thermocouple may then be determined as a function of either the
heating time, or the cooling time, or both.
[0014] In a further aspect of the invention the measurement range
of the thermocouple vacuum sensor may be significantly extended.
This may be achieved by utilizing the effect that both heat
capacity and thermal conductivity of the gas surrounding the
thermocouple determine the heating and cooling time. The extension
of the measurement range at higher pressures is based on the
increasing influence of the heat capacity of a gas, which changes
with pressure at the upper measurement range. To evaluate heat
capacity at high pressure a high amount of heating energy is
required for the thermocouple to be heated from its lower threshold
temperature to its upper threshold temperature at the upper end of
the measuring range.
[0015] As both thermal conductivity and heat capacity decrease with
pressure only a small amount of heating energy is required for the
thermocouple to be heated from its lower threshold temperature to
its upper threshold temperature at the lower end of the measuring
range. To increase the sensor's sensitivity at low pressure the
power applied to the thermocouple may be reduced. The reduced power
increases the heating time, thereby improving the sensor's
resolution and extending its measurement range at low pressure.
[0016] In yet another aspect of the invention variable heating
power may be applied to the thermocouple from an electronic heating
circuit during a measuring cycle. To increase sensitivity at low
pressure while maintaining the ability to measure high pressure the
heating power applied to the thermocouple during the heating phase
may increase over time until the upper temperature threshold is
reached. The increase in power may be achieved by increasing the
voltage of heating pulses applied to the thermocouple with a
constant duty cycle, or by increasing the duty cycle of heating
pulses having a constant voltage, or by a combination thereof.
[0017] At low pressure a small amount of electrical energy is
sufficient to heat the thermocouple from the lower to the upper
temperature threshold. Ramping up the power applied to the
thermocouple over time causes the time to collect the necessary
energy to reach the upper temperature threshold to increase. For a
given time resolution of the time measuring device, this "time
extension" may be used to achieve better resolution for the partial
vacuum at lower pressure ranges.
[0018] In contrast to operating methods which maintain a constant
high temperature of the thermocouple, the variable temperature
operating method according to one aspect of the invention operates
at high temperature only during short periods of time at the end of
a measuring cycle. The average temperature of and average power
applied to the thermocouple utilizing a variable temperature
operating method may be significantly lower than those of a
thermocouple operating at constant temperature. This is
advantageous especially when measuring pressure of small gas
volumes, for example in analytical instruments, to limit the power
applied to the gas and thereby the influence that measuring its
pressure has on the gas in the probe.
[0019] The following detailed description of the invention is
merely exemplary in nature and is not intended to limit the
invention or the application and uses of the invention.
Furthermore, there is no intention to be bound by any theory
presented in the preceding background of the invention or the
following detailed description of the invention.
BRIEF DESCRIPTION OF THE DRAWINGS
[0020] FIG. 1 shows a diagram of a thermocouple system in a vacuum
measuring tube operatively connected to an operating circuit
according to an aspect of the invention.
[0021] FIG. 2a and FIG. 2b illustrate exemplary temperature
profiles of the thermocouple system as in FIG. 1 when operated at
different pressure levels.
[0022] FIG. 2a illustrates a temperature profile at higher
pressure.
[0023] FIG. 2b illustrates a temperature profile at low
pressure.
[0024] FIG. 3a shows an exemplary embodiment of an electronic
circuit for operating the thermocouple system as in FIG. 1.
[0025] FIG. 3b shows an alternative embodiment of an electronic
circuit for operating the thermocouple system as in FIG. 1.
[0026] FIG. 4 illustrates alternative methods of operating a
thermocouple vacuum measuring tube according to an aspect of the
invention, showing voltages applied to the thermocouple and the
resulting temperatures of the thermocouple over time.
DETAILED DESCRIPTION
[0027] Referring to FIG. 1, a block diagram of an exemplary system
in which the principles of the present invention may be
advantageously practiced is illustrated generally. A thermocouple
sensor 101, e.g. a commercially available vacuum measuring tube
model DV-4 or DV-6 manufactured by Teledyne Hastings, is
operatively connected to an operating circuit through connections
A, B, and C. A variable temperature operating method is used
wherein the temperature at the thermocouples continuously cycles
between a lower threshold .theta..sub.I and an upper threshold
.theta..sub.II. This is achieved by modifying a traditional
constant temperature sensor system such that an alternating heating
voltage which is applied to a series connection of two
thermocouples can be selectively turned on and off through switches
106 and 107.
[0028] In the exemplary circuit thermoelectric voltage output B is
operatively connected to an amplifier 102. The output of amplifier
102 is connected in parallel to a first comparator 103 having
switching threshold I, and to a second comparator 104 having
switching threshold II. The outputs of comparators 103 and 104 are
operatively connected to a logic circuit 105. Logic circuit 105
controls switches 106 and 107, which activate and deactivate the
heating of thermocouple sensor 101. When the amplified
thermoelectric voltage exceeds threshold II comparator 104 provides
an "off" signal to logic circuit 105. Logic circuit 105 responsive
to receiving the "off" signal controls switches 106 and 107 open.
When switches 106 and 107 are opened heating voltage is removed
from the thermocouple sensor 101 and thermocouple sensor 101 cools
down.
[0029] When the amplified thermoelectric voltage falls below
switching threshold I comparator 103 provides an "on" signal to
logic circuit 105, which responsive thereto controls switches 106
and 107 to be closed. When switches 106 and 107 are closed heating
current is applied from the secondary winding of transformer 108 to
thermocouple sensor 101 which causes thermocouple sensor 101 to
heat up. Alternating voltage is applied to the primary winding of
transformer from an alternating voltage source.
[0030] Reference voltages I and II used in comparators 103 and 104
to establish switching thresholds I and II are adjustable using
voltage dividers.
[0031] The electronic circuit as described causes a continuous
heating-cooling cycle in which the temperature of the thermocouple
sensor alternates between .theta..sub.I, and .theta..sub.II, said
temperatures corresponding to switching threshold voltages I and
II.
[0032] FIG. 2a and FIG. 2b show two exemplary temperature profiles
for a measuring system as in FIG. 1. Beginning at a starting
temperature 211, the thermocouple sensor is heated until its
temperature reaches the upper temperature threshold .theta..sub.II.
The thermocouple sensor then cools off until it temperature reaches
the lower temperature threshold .theta..sub.I. The first measuring
cycle begins when the temperature falls to .theta..sub.I.
[0033] FIG. 2a illustrates an exemplary temperature profile typical
for higher pressure. Due to the high heat capacity and high thermal
conductivity of a gas at high pressure surrounding the thermocouple
sensor, the temperature rises only slowly from threshold
.theta..sub.I to .theta..sub.II. It takes a time t.sub.Ha to heat
the thermocouple sensor from temperature .theta..sub.I to
.theta..sub.II. Respectively, once the heating power to the
thermocouple sensor is switched off, it cools off quickly due to
high heat dissipation via the gas. The time to cool off from
.theta..sub.I to .theta..sub.II is t.sub.Ca.
[0034] In contrast, FIG. 2b illustrates an exemplary temperature
profile typical for low pressure. Starting at the lower threshold
temperature .theta..sub.I the thermocouple sensor when heated
quickly reaches the upper threshold temperature .theta..sub.II
after time t.sub.Hb. However, at low pressure it takes the
thermoelectric sensor a longer time t.sub.Cb to cool off than at
higher pressure.
[0035] Thus, pressure in a partial vacuum can be determined by
measuring either the heating time t.sub.H, or the cooling time
t.sub.C, or both, and calculating the pressure as a function of
t.sub.H, or t.sub.C, or both t.sub.H and t.sub.C.
[0036] FIG. 3a and FIG. 3b show two exemplary electronic circuits
capable of generating temperature profiles as shown in FIG. 2a and
FIG. 2b.
[0037] In the exemplary embodiment illustrated in FIG. 3a
thermocouple sensor 311 or alternatively a single thermocouple 320
is connected to an electronic heating circuit comprising power
source 318 and switch 317. The power source may deliver constant
voltage or constant current or alternating voltage or alternating
current. During a heating period switch 317 is pulsed on-off
thereby generating heating pulses which raise the temperature of
thermocouple sensor 311. Heating pulses generated during a heating
period may be of equal duration throughout the heating period.
Heating pulses may also change in duration over time, e.g. starting
with a minimum pulse duration at the beginning of a heating cycle
and gradually increasing to a maximum pulse duration at the end of
a heating cycle.
[0038] During the off-time of the heating pulses, i.e. when switch
317 is open, switch 316 closes, thereby connecting thermocouple
sensor 311 or 320 to a sample-and-hold-circuit having an amplifier
312. Switch 316 and the sample-and-hold circuit comprising
amplifier 312, a capacitor and two resistors, form a voltage
sensing circuit.
[0039] The output of amplifier 312 is connected to the inputs of
comparators 313, which establishes the lower temperature threshold
I, and 314, which establishes the upper temperature threshold II.
Comparators 313 and 314 may be stand-alone components or integrated
into a larger control device, e.g. into a microcontroller 315.
[0040] Microcontroller 315 processes the signals from the
comparators and controls switch 317. Switch 317 is opened and
closed in a pulse-like manner as long as the amplified
thermoelectric voltage out of amplifier 312 has not yet reached the
upper threshold II. Switch 317 is opened when the amplified
thermoelectric voltage at amplifier 312 exceeds threshold II, and
is not closed again until the thermoelectric voltage falls below
threshold I.
[0041] One measuring cycle comprises a heating period during which
the thermocouple sensor 311 or 320 is heated from a lower
temperature threshold .theta..sub.I to an upper temperature
threshold .theta..sub.II, and a cooling period during which switch
317 is opened and the thermocouple sensor 311 or 320 cools off from
temperature .theta..sub.II to .theta..sub.I.
[0042] Microcontroller 315 may measure and store in memory any
combination of following values indicative of the heating time: The
number of heating pulses within a measuring cycle, the aggregated
heating time of all heating pulses in a measuring cycle, and the
total duration of the heating period. Microcontroller 315 may also
measure and store the duration of the cooling period within a
measuring cycle.
[0043] Microcontroller 315 may calculate the pressure of the
partial vacuum surrounding thermocouple sensor 311 or 320 by
comparing the heating time, or the cooling time, or both, with
reference data stored in a memory associated with the
microcontroller. The reference data may comprise data organized in
tables, which have been populated using an initial calibration
procedure. Microcontroller 315 may communicate the output of its
pressure calculation as an analog voltage, e.g. by generating a
pulse-width modulation (PWM) signal, which is smoothed using a
low-pass filter and outputted via the output amplifier 319. Many
other forms of communicating data from a microcontroller to another
device are known and could be used, e.g. serial data communication
such as a UART interface, parallel data communication, or wireless
data communication.
[0044] FIG. 3b shows an alternative exemplary electronic circuit
similar to that in FIG. 3a. Unlike FIG. 3a in which heating pulses
to the thermocouple sensor 311 or 320 are provided by a constant
voltage or constant current out of power supply 318 the circuit in
FIG. 3b operates with a variable heating voltage or current. The
heating voltage or current supplied to thermocouple sensor 321
ramps up over time during a measurement cycle, creating a pulsed
sawtooth voltage profile at thermocouple sensor 321.
[0045] Power source 328 first supplies power to a
resistor-capacitor combination 330. A switch 327 which opens and
closes cyclically is controlled by the microcontroller 325. The saw
tooth ramp thus generated is applied to the impedance transformer
331.
[0046] When switch 332 is momentarily closed by the microcontroller
a heating pulse whose voltage level corresponds to the
instantaneous value of the sawtooth ramp is generated. The
switching-on time for the heating pulse at the switch 332 is much
smaller than the time for which the switch 327 remains open. Switch
327 is open until SPI is reached.
[0047] The remaining components of circuit 3b operate equivalent to
their counterparts in FIG. 3a. Switch 326 corresponds to switch
316, sample-and-hold circuit with amplifier 322 corresponds to 312,
comparators 323 and 324 correspond to 313 and 314 and output
amplifier 329 corresponds to 319.
[0048] Neither of the two electronic circuits shown in FIG. 3a and
FIG. 3b requires use of an analog-to-digital converter. The
microcontroller 315, 325 may communicate the measured pressure by
selecting a duty cycle of its PWM output feeding into output
amplified 319, 329. The duty cycle may be determined through a
look-up-table or characteristic curve which associates heating and
cooling times with a PWM output duty cycle.
[0049] FIG. 4 shows exemplary pulse shape diagrams and temperature
curves generated by the two electronic circuits according to FIG.
3a and FIG. 3b.
[0050] FIG. 4a shows an exemplary diagram of voltage over time that
is applied to the thermocouple sensor. As shown the heating pulses
have a constant pulse length and a constant voltage level. The
corresponding temperature of the thermocouple sensor over time is
illustrated in FIG. 4b. As shown the temperature cycles between the
lower threshold temperature .theta..sub.I and the upper temperature
threshold .theta..sub.II.
[0051] FIG. 4c shows an alternative diagram of voltage over time
that is applied to the thermocouple sensor. As illustrated the
length of the heating pulses continuously increases throughout the
heating period.
[0052] FIG. 4d shows an exemplary diagram of voltage over time that
may be generated by the electronic circuit illustrated in FIG. 3b.
Here the voltage of each heating pulse increases throughout the
heating period following sawtooth ramp.
[0053] The heating pulses illustrated in both FIG. 4c and FIG. 4d
may cause a temperature profile of the thermocouple sensor over
time as shown in FIG. 4e. Early in the heating period the
temperature of the thermocouple sensor rises slower when pulses
shown in FIG. 4c or FIG. 4d are used than when pulses as in FIG. 4a
are used. This is because the average power applied to the
thermocouple sensor by heating pulses 4c or 4d in the early heating
phase is lower than that by heating pulses 4a. The total heating
time using heating pulses 4c or 4d is longer than the total heating
time using pulses 4a. A longer heating time is advantageous in
particular at low pressure as it increases the resolution of the
time measurement at a given microcontroller operating
frequency.
[0054] Microcontroller 315, 325 may have an associated data memory
in which tables are stored, which for typical thermocouple sensors
are determined in a one-time basic calibration procedure. In these
tables an additional time for a PWM output pulse (for a constant
pause time of the PWM signal) or, for each time, for a PWM output
pulse and for a pause in the PWM output signal, is associated with
the measured time interval.
[0055] The characteristic line or look-up table in microcontroller
315, 325 which determined the controller's PWM output as a function
of the measured heating time, cooling time, or both, may be
manually calibrated, e.g. through one of more calibration switches
operatively connected to the microcontroller. Also, predetermined
calibrations for known mass-produced thermocouple sensors may be
stored in the micro controller's memory.
[0056] Manual calibration may also be used to compensate aging,
e.g. caused by deposits formed on a thermocouple sensor. For this
purpose it is necessary to store only one time interval each
(heating time, cooling time), which is determined by the circuit in
a first keystroke and a first (upper) reference pressure, and a
second keystroke at a second (lower) reference pressure, in a
memory which is associated with the corresponding table. If no
calibrated comparison instruments are available for the reference
pressure, it is also possible for a pressure above the higher
measurement range limit (first reference pressure), to adjust to
the higher measurement range limit, and for a pressure less than
the lower measurement range limit (second reference pressure), to
adjust to the lower measurement range limit.
[0057] While the present invention has been described with
reference to exemplary embodiments, it will be readily apparent to
those skilled in the art that the invention is not limited to the
disclosed or illustrated embodiments but, on the contrary, is
intended to cover numerous other modifications, substitutions,
variations and broad equivalent arrangements that are included
within the spirit and scope of the following claims.
* * * * *